Balanced radioreceiver



p 3, 1929- J. D. R. FREED BALANCED RADIORECEIVER Filed July 19., 1924 hmmv IN ZEN [O fled 1% I ATTORNEYS Patented Sept. 3, 1929.

UNITED STATES PATET OFFICE. I

JOSEPH DAVID ROTH FREED, OF WOODMERE, NEW YORK, ASSIGNOR TO FREED- EISEMANN RADIO CORPORATION, OF BROOKLYN, NEW YORK, A CORPORATION OF NEW YORK.

BALANCED RADIORECEIVER.

Application filed July 19,

to inductive or electromagnetic coupling between certain of the coils, or it may be due to electrostatic coupling between certain of the elements, or it may be due to a comb1nation of both of these effects.

Numerous methods and arrangements have been proposed for eliminating such coupling effects and especially that which is due to the inherent capacity of the grid and plate elec trodes of the audion tube.

conditions which avoid electromagnetic coupling or reduce such electromagnetic coupling below the point at which it is capable of causing the generation of oscillations,

the various circuits can be tuned to resonance for the purpose of employing, for exan'iple, radio frequency amplification 1n the most efficient manner.

The principle underlying the invention avoids the necessity of carefully adjusting the electromagnetic potentials developed and utilized in neutralizing circuits heretofore employed and thus greatly simplifies the manufacture and adjustment of the various parts with respect to each other.

In carrying out my invention I assemble the various elements of the receiving circuit in such manner that the capacity relation between the circuits and elements external to the audioii tube is opposite to and distinctly in excess of the capacity effect between the plate and grid electrodes of the audion, i. e. the internal capacity of the audion.

This is readily accomplished, for

example, by disposing the effective areas of the fixed condenser plates of successive re onant grid circuits with respect to each other, so that one grid circuitoifers to the other grid circuit a capacity coupling which is in the reverse sense to that existing between the plate and grid electrodes of-the first of the two audio is, and distinctly greater in value. The difirere"ee or net ca peel-lay than r adily r :luced to tern 1924. Serial No. 726,997.

by an adjustable capacity introduced di rectly between the grid and plate electrodes of that tube.

The invention will be more clearly understood by reference to the following detailed description and accompanying drawings.

In these drawingsz- Figure 1 represents the conventional circuit diagram of a detector circuit preceded by one stage of tuned radio frequency amplification; and

Figure 2 represents a perspective View of a number of tuning condensers arranged in conventional manner on the backof an instrument panel.

Referring to Figure 1, 1 represents a receiving antenna grounded at 3 and connected to an auto-transformer 2 which at the time forms the inductance of an oscillatory circuit containing the variable condenser i. 5 represents a three electrode therinionic tube containing the filament F, grid G and plate P. Thestationary plates of condenser 4i denoted s are connected to the grid of tube 5, and the movable plates are connected to the filament F. It will be noted that thus the movable plates, as is customary in radio receiving circuits, are grounded. The filament F is supplied with heating current from the battery A, whose current is controlled by the rheostat 8. The plate circuit of tube 5 includes the primary coil of a radio frequencytransformer 6 and also includes the battery B and the filament F.

The secondary coil of transformer 6 is in circuit with the tuning condenser 7 and, similar to the previously described oscillatory circuit 2, 4;, is tuned to the frequency desired to be received. The stationary plates 8 of condenser 7 are connected by way of a grid condenser 9 and grid leak 10 shunting the latter, to the grid G of detector tube D. The movable plates of condenser 7 are connected to the filament of tube D. Filament F is likewise supplied with current from battery A and the movable plates of condenser 7 like those of condenser 4 are grounded in this particular case by way of the filament battery lead 11. The plate filament circuit of tube D comprises the plate P, telephone 12, battery B and the filament F, a bridging condenser 13 of suitable value being provided between the plate 1? and filament F to afford path for the high frequency oscillations amend the telephone condenser and battery B A telephone T is connected in parallel to telephone condenser 12 in conventional manner. The primary and secondary windings of radio frequency transformer 6 are wound and connected in circuit so that the lower ends of both windings in the diagram have like polarity, "for instance for a given instant a polarity. This would imply for this instant a polarity at the upper end of the secondary coil of transformer 6 and thus a polarity of the stationary plates of condenser 7. The lower end of the primary of transformer 6 having a polarity at that instant would result in a polarity of plate P of tube 5. This being the case, the stationary plates 8 of condenser 4 will at that moment be subjected to a negative charge due to the capacity between plate and grid of tube 5. The stationary plates 8 of condenser 7, being connected to the upper end of the secondary of transformer 6, will at this same moment become negative and will impart a plus charge to stationary plates 8 of condenser 4 and thus to the grid of tube 5, exactly equal in value to the negative charge imparted thereto by the capacity effect of the plate and grid electrodes of tube 5, assuming for the moment that the internal tube capacities are equal to the external capacities of the circuit. Thus the voltage oi the grid of tube 5, which is connected with the stationary plates of condenser l, will not be 'afiected by'the reactance voltages set up by the change in current strength in the primary and secondary coils of transformer 6, and therefore, feedback does not occur.

As has been stated hereinbefore, in tube circuits of this character undesired oscillations are produced, due to the inherent capacity between the grid and plate of the thermionic tubes. In this particular instance, the inherent capacity between the grid and plate of tube 5 is indicated symbolically in dotted lines by the condenser Ct, and the circuit coupled by capacity Ct would include the plate of tube 5, primary of transformer 6, battery B, filament lead 11, condenser 4 and auto-transformer 2 and the grid G of tube 5. Oscillations will occur in this case it the grid-plate, or internal capacity of the tube is greater than the external capacity 01' the circuit, denoted with CX and symbolically drawn in dotted lines in Figure 1.

In order to carry out the idea underlying my present invention, 1 take advantage of the fact that the capacities between the stationary plates 8 of condensers 4: and 7, for the sizes ordinarily used in radio receiving sets and at the distances at which these condensers can easily be arranged from each other, are considerably larger than the inherent capacities ofthe grid and plate of the tube located in the circuits between these condensers. These condenser plates, together with the other circuit elements, such as wiring, form the external capacity of the circuit with relation to the internal gridplate capacity of the tube. The other plates of these condensers, being grounded as shown, may be eliminated from this consideration. As a consequence, under the last-named conditions undesired oscillations are produced also, but in this case they are not due to the internal capacity coupling of the tube 5 through the grid and plate, but

i are due to the external capacity coupling of the aforementioned character.

I have indicated in Figure 1 in dotted lines symbolically the external capacity coupling, especially that between the stationary plates 8 of tuning condensers at and 7 by the condenser C The polarity signs applied to condenser C represent the voltages set up between the two grid circuits as a result of the connection of the grid of tube 1) to the then negative end of the secondary coil of transformer 6. The polarity signs indicated with reference to condenser C represents the voltages set up by reason of the plate of tube 5 being connected with the momentarily positive end of the primary coil of transformer 6. Since obviously the voltage relations of the two capacities G and Ct are in opposition to each other, and since the capacity value CX is greater than 0,, it follows that the effect of capacity C will preponderate on the grid gt tipbe 5 and would lead to undesired feed- In order to balance the excess of ca aacity C over capacity (1,, I arrange in para lel to the grid and plate of tube 5, in other words, in parallel to the capacity (1,, an adjustable condenser C Since the tube capacity C, is very small, the excess capacity to be added to the tube capacity in order to balance capacity O may be of appreciable size, so'that it can be easily adjusted to a sufficient nicety to properly balance the tube capacity against the capacity C This method of preventing feed-back is superior to those heretofore proposed, in which the electrostatic coupling has been due to the inherent tube capacity and has been eliminated by arranging an adjustable capacity in series with an inductance to oppose the tube capacity coupling. As has been stated hereinbefore, the tube capacity being very small, an auxiliary condenser which is of necessity of the same order of magnitude is also very small and consequently difiicult to adjust to the required nicety in order to completely balance the tube capacity.

I have shown in Figure 1 the manner in which the inherent capacity eii ect of one tube may be balanced against the external capacity of the circuit. It is obvious that this arrangement may be repeated in a multistage circuit as many times as required. For instance in Figure 2 I have shown three condensers arranged in conventional manner on the back of an instrument panel, 4 and 7 representing the correspondingly numbered condensers in Figure 1. 14; indicates a condenser similar to condenser 4 and located similarly in a radio frequency stage preceding the one including tube 5. I have indicated in dotted lines the capacities C existing between the stationary plates 8 of adjacent condensers. The amplifier tubes, being located respectively in circuits between adjacent condensers, are such provided with a suitable condenser C such as is shown in Figure 1, placed in parallel to the grid and plate of the tube.

I claim 1. An amplifier circuit arrangement including a plurality of thermionic tubes, each having a filament, a grid and a plate in suitable circuit connection, a transformer between adjacent tubes, having its primary coil in the plate circuit ofthe first'of Said adjacent tubes and its secondary coil in the grid circuit of the succeeding tube, a condenser in the grid circuit of each tube for tuning said circuit to the desired frequency, the capacity relation between the'condenser portions connected to the grids of said adjacent tubes being dimensioned opposite to and distinctly in excess of the internal capacity effect between the grid and plate of the first of said adjacent tubes and a capacity element in parallel to said grid and plate for neutralizing said excess of capacity.

2. An amplifier circuit, arrangement including a plurality of thermionic tubes, each having a filament, a grid and a plate in suitable circuit connection, a transformer be tween adjacent tubes, having its primary coil in the plate circuit of the first of said adj acent tubes and its secondary coil in the grid circuit of the succeding tube, a condenser in the grid circuit of each tube for tuning said circuit to the desired frequency, the capacity relation between the condenser portions connected to the grids of said adjacent tubes being dimensioned opposite to and distinctly in excess of the internal capacity effect be tween the grid and plate of the first of said adjacent tubes and an adjustable capacity element in parallel to said grid and plate for neutralizing said excess of capacity.

v3. An amplifier circuit arrangament including a plurality of thermionic tubes, each having a filament, a grid and a plate in suitable circuit connection, a transformer be tween adjacent tubes, having its primary coil in the plate circuit of the first of said adjacent tubes and its secondary coil in the grid circuit of the succeeding tube. a condenser in the grid circuit of each tube for tuning said circuit to the desired frequency, said transformer coils being suitably wound and connected into their respective circuits to establish a potential at the condenser portion connected to the grid of the second tube opposite to that existing at the condenser portion connected to the grid of the first tube, the capacity between said condenser portions being dimensioned in excess of the capacity between said grid and plate, and a capacity element in parallel to said grid and plate for neutralizing said excess capacity.

4. An amplifier circuit arrangement including a plurality of thermionic tubes, each having a filament, a grid and a plate in suitable circuit connection, a transformer between adjacent tubes, having its primary coil in the plate circuit of the first of said adj acent tubes and its secondary coil in the grid circuit of the succeeding tube, a condenser in the grid circuit of each tube for tuning said circuit to the desired frequency, said transformer coils being suitably wound and connected into their respective circuits to establish a potential at the condenser portion connected to the grid of the second tube opposite to that existing at the condenser portion connected to the grid of the first tube, the capacity between said condenser portions being dimensioned in excess of the capacity between said grid and plate, and an adjustable condenser in parallel to said grid and late for neutralizing said excess capacity.

I JOSEPH DAVID ROTH FREED. 

